A three-dimensional laser scanner can be used to survey an environment such as a process plant, vessel or other facility. A typical scanner includes a laser rangefinder which can measure a distance between the scanner and a point on a surface which is in view. By sweeping through a field of view (typically 360 degrees horizontally and nearly 180 vertically), the scanner can capture a set of ranges (herein referred to as “laser scan data”) for the surrounding environment. These can be used to generate a set of points in three-dimensional space, often referred to as a “point cloud”. An example of a point cloud is described in EP 1 176 393 A2.
Multiple scans can be performed at different positions in an environment and point clouds from different scans can be combined to produce a combined (or “aggregated”) point cloud covering a wider area. An example of combining point cloud data can be found in WO 2004/003844 A1.
In addition to acquiring range data, the scanner can also capture images of the surrounding environment by measuring intensity of reflected laser light or using a camera.
The point cloud(s) and images can be used to visualize and/or analyze an environment using a point cloud viewer application or a three-dimensional computer-aided design (CAD) application. Typically, these applications fall into two categories, namely those that work with points from individual scans and those that work with points combined from multiple scans.
One of the simplest applications of laser scanning is to display an image captured by an individual scan. Because the image from a laser scan is spherical, covering the area around the laser scanner, the software application can map the image onto the inside of a sphere. The application can display a portion of the sphere on a computer screen. The user can rotate the view in order to view different portions of the entire image. This presentation is called a “bubble view”.
In bubble view, the user can select a spot on the image and retrieve the three-dimensional coordinate of that location using the point cloud data for that laser scan. By selecting two points, the user can measure distances.
One type of application can overlay a three-dimensional CAD model in a bubble view. Because the application knows the three-dimensional location of the points in bubble view, it can obscure the appropriate portions of the CAD model behind the bubble view. This combined image can be useful when designing new areas of the facility.
An appealing feature of a bubble view is that it looks realistic. Realism comes from the image captured at the scanner location. A limitation of bubble views is, however, that they can only be produced for the locations at which a laser scanner was positioned. A user can select a bubble view and rotate to the left and right or up and down, but he/she cannot move forward, backward, horizontally or vertically in order to view the environment from a different perspective.
To allow free roaming, some software applications work with a combined point cloud from multiple scans. Using such an application, a user chooses a location within a facility and a viewing direction. The application then displays each point in the combined point cloud around that location from the point of view of the user. The user can move the viewing location and direction to see the points from different perspectives.
Some applications can display a CAD model in the same three-dimensional space as the combined point cloud. A user can then measure distances between locations in the CAD model and points in the point cloud. The user can also determine if portions of the point cloud intersect portions of the CAD model.
Although displaying a combined point cloud allows the user to view points from more than one perspective, this approach can have one or more drawbacks.
Displaying individual points tends to be computationally expensive. Gaps can appear in a representation of a scanned surface at close distances and so it can become difficult to discern the surfaces.